1. Field of the Invention
This invention relates to a power converting apparatus using self turn-off type semiconductor devices.
2. Description of the Related Art
FIG. 21 is a circuit diagram, showing a conventional power converting apparatus. In this apparatus, an AC voltage supplied from a first AC system 1 is transformed by a first transformer 2, and the transformed voltage is applied to a first converter 3A, where it is converted to a DC voltage.
The first converter 3A has self turn-off type semiconductor devices, e.g., gate turn-off thyristor (hereinafter referred to as "GTO") switches 69-74, and di/dt suppressor circuits 75-80 connected in series to the switches 69-74, respectively. Each of the GTO switches 69-74 consists of a GTO 93, diodes 94 and 95, a capacitor 96, and a resistor 97. Each of the suppressor circuits 75-80 consists of a diode 99, a resistor 100, and a reactor 98.
The DC voltage obtained by the converter 3A is smoothed by a capacitor 68, and is converted again to an AC by a second converter 5A, and is applied to a second AC system 7 after being transformed by a second transformer 6.
Like the first converter 3A, the second converter 5A consists of GTO switches 81-86 and di/dt suppressor circuits 87-92 connected to the switches. Each element has a structure similar to that of the first converter 3A.
In the structure as above, the diode 94 serves as a free-wheel diode for feeding back a circuit current obtained when the GTO switch 72 is turned off. The diode 95 serves to guide a current, having been flowing through the GTO 93 before the GTO 93 was turned off to the capacitor 96.
The capacitor 96 serves as a so-called snubber condenser for suppressing below an allowable value the rate dv/dt of the increase of the forward voltage of the GTO 93 which will occur as a result of turn off thereof. The resistor 97 is discharge resistor which relieves the charge of the capacitor 96 when the GTO 93 is turned on. These elements 93-97 constitutes the GTO switch 69.
The reactor 98 suppresses below an allowable value the rate di/dt of the increase of the forward current of the GTO 93 which will occur as a result of turn on thereof. The diode 99 serves to guide a current, having been flowing through the reactor 98 before the GTO 93 was turned off, to the resistor 100. The electromagnetic energy trapped by the reactor 98 is consumed by the resistor 100.
The above-described conventional device, however, has the following disadvantages:
(1) The charges of the capacitor 96 charged when the GTO 93 is in the off-state, will be discharged by the resistor 97 when the GTO 93 is turned on, resulting in heat loss. Thus, the efficiency of the power converting apparatus is deteriorated.
(2) The current flowing through the reactor 98 when the GTO 93 is turned on, is discharged by the resistor 100 when the GTO 93 is turned off, resulting in heat loss. Thus, the efficiency of the power converting apparatus is deteriorated.
(3) For example, if the GTO switch 72 is erroneously turned on where the GTO switch 69 is in the on-state, the DC circuit will short. Thus, it is difficult to protect the apparatus from excessive current. The apparatus must have a complicated structure.